Conventionally, an image-forming method using a so-called photosensitive toner functioning as both a developer and a photoconductor is known. In such a known method, a thin layer of the toner having a photoconductivity is formed on a conductive drum, and the toner thin layer is then exposed to light to obtain a toner image.
FIG. 2 shows an example of the prior art image-forming method. Referring to FIG. 2, a toner thin layer 23 of a photosensitive toner 22 charged to have a given polarity by triboelectric charging or the like is formed on a toner retaining member 21 having a conductivity (the photosensitive toner 22 may be charged to have a given polarity by corona charging or the like after forming the toner thin layer). Then, exposure 24 according to an original image is applied to the toner thin layer 23. Accordingly, the toner is made conductive by the exposure 24, and the charge possessed by the toner is dissipated through the conductive toner retaining member 21 which is grounded, or a charge having a reverse polarity is injected into the toner. Thus, an electrostatic latent image 25 is formed on the toner thin layer. On the other hand, a transfer material 26 is brought into contact with the toner thin layer on which the latent image 25 has been formed, and a corona ion is irradiated from a back side of the transfer material 26 by means of a transfer charger 27. As a result, the transfer material 26 is charged to have a polarity reversed to or the same as that of the toner not exposed, thereby transferring the toner onto the transfer material 26. In the former case where the transfer material is charged to have the reversed polarity, a positive image is formed, while in the latter case where the transfer material is charged to have the same polarity, a negative image is formed.
However, the above-mentioned transfer step is different from a general transfer process for transferring a toner image only formed on a photosensitive body in a xerography system which is represented by a Carlson process. That is, the transfer step as shown in FIG. 2 includes contacting of the transfer material with the toner thin layer and separating of the toner (toner image) having a given polarity only from the toner thin layer. Generally, the adherence between the toner and the transfer material is lacking, and it is accordingly difficult to sufficiently contact the transfer material with the toner thin layer, thus making an improvement in transfer efficiency difficult.
In the xerography system as mentioned above wherein a toner image only to be transferred is formed on the photosensitive body, there has been proposed that a presser means for pressing the transfer material is provided on an upstream side of a transfer region into which the transfer material is introduced or on a downstream side of the transfer region from which the transfer material is discharged, so as to improve the contact between the transfer material and the toner on the toner retaining member (e.g., a photosensitive drum). However, since the adherence between the transfer material and the toner is low, the presser means as mentioned above cannot yet provide a sufficient contact between the transfer material and the toner thin layer (toner image) at an opening portion of the transfer charger located at the transfer region. Therefore, the image density cannot be increased.